For T cells to respond to foreign polypeptides, at least two signals must be provided by antigen-presenting cells (APCs) to resting T lymphocytes (Jenkins, M. and Schwartz, R. (1987) J. Exp. Med. 165:302-319; Mueller, D. L. et al. (1990) J. Immunol. 144:3701-3709). The first signal, which confers specificity to the immune response, is transduced via the T cell receptor (TCR) following recognition of foreign antigenic peptide presented in the context of the major histocompatibility complex (MHC). The second signal, termed costimulation, induces T cells to proliferate and become functional (Lenschow et al. (1996) Annu. Rev. Immunol. 14:233). Costimulation is neither antigen-specific, nor MHC-restricted, and is provided by distinct cell surface molecules expressed by APCs (Jenkins, M. K. et al. (1988) J. Immunol. 140:3324-3330; Linsley, P. S. et al. (1991) J. Exp. Med 173:721-730; Gimmi, C. D. et al. (1991) Proc. Natl. Acad Sci. USA 88:6575-6579; Young, J. W. et al. (1992) J. Clin. Invest. 90:229-237; Koulova, L. et al. (1991) J. Exp. Med. 173:759-762; Reiser, H. et al. (1992) Proc. Natl. Acad Sci. USA 89:271-275; van-Seventer, G. A. et al. (1990) J. Immunol. 144:4579-4586; LaSalle, J. M. et al. (1991) J. Immunol. 147:774-80; Dustin, M. I. et al. (1989) J. Exp. Med. 169:503; Armitage, R. J. et al. (1992) Nature 357:80-82; Liu, Y. et al. (1992) J. Exp. Med 175:437-445).
The proteins B7-1 (CD80) and B7-2 (CD86) are critical costimulatory molecules (Freeman et al. (1991) J. Exp. Med. 174:625; Freeman et al. (1989) J. Immunol. 143:2714; Azuma et al. (1993) Nature 366:76; Freeman et al. (1993) Science 262:909). B7-2 plays a predominant role during primary immune responses, while B7-1, which is upregulated later during an immune response, may be important for prolonging primary T cell responses or costimulating secondary T cell responses (Bluestone (1995) Immunity 2:555).
CD28 is a ligand for both B7-1 and B7-2 that is constitutively expressed by resting T cells and increases in expression following T cell activation. Ligation of CD28 in conjunction with a TCR signal results in transduction of a costimulatory signal that induces T cells to proliferate and secrete IL-2 (Linsley, P. S. et al. (1991) J. Exp. Med. 173:721-730; Gimmi, C. D. et al. (1991) Proc. Natl. Acad. Sci. USA 88:6575-6579; June, C. H. et al. (1990) Immunol. Today 11:211-6; Harding, F. A. et al. (1992) Nature 356:607-609). A second B7-1 and B7-2 ligand, CTLA4 (CD152), is homologous to CD28 but not expressed by resting T cells. CTLA4 expression occurs following T cell activation (Brunet, J. F. et al. (1987) Nature 328:267-270). Ligation of CTLA4 results in transduction of an inhibitory signal that prevents T cell proliferation and cytokine secretion. Thus, CTLA4 is a critical negative regulator of T cell responses (Waterhouse et al. (1995) Science 270:985) (Allison and Krummel (1995) Science 270:932). The third member of the CD28 family to be discovered is ICOS (Hutloff et al. (1999) Nature 397:263; WO 98/38216). Ligation of ICOS by its ligand (ICOS-L) results in high levels of cytokine expression, but limited T cell expansion (Riley J. L. et al. (2001) J. Immunol. 166:4943-48; Aicher A. et al. (2000) J. Immunol. 164:4689-96; Mages H. W. et al. (2000) Eur. J Immunol. 30:1040-7; Brodie D. et al. (2000) Curr. Biol. 10:333-6; Ling V. et al. (2000) J. Immunol. 164:1653-7; Yoshinaga S. K. et al. (1999) Nature 402:827-32). If T cells are stimulated through the T cell receptor in the absence of a costimulatory signal, they become nonresponsive, anergic, or die.
The importance of the B7:CD28/CTLA4/ICOS costimulatory pathway has been demonstrated in vitro and in several in vivo model systems. Blockade of this costimulatory pathway results in the development of antigen specific tolerance in murine and human systems (Harding, F. A. et al. (1992) Nature 356:607 609; Lenschow, D. J. et al. (1992) Science 257:789 792; Turka, L. A. et al. (1992) Proc. Natl. Acad. Sci. USA 89:11102 11105; Gimmi, C. D. et al. (1993) Proc. Natl. Acad. Sci. USA 90:6586 6590; Boussiotis, V. et al. (1993) J. Exp. Med. 178:1753 1763). Conversely, expression of B7 by B7-negative murine tumor cells induces T-cell mediated specific immunity accompanied by tumor rejection and long lasting protection to tumor challenge (Chen, L. et al. (1992) Cell 71:1093 1102; Townsend, S. E. and Allison, J. P. (1993) Science 259:368 370; Baskar, S. et al. (1993) Proc. Natl. Acad. Sci. 90:5687 5690.). Therefore, manipulation of the costimulatory pathways offers great potential to stimulate or suppress immune responses in humans.
The discovery of more members of the B7-1 and CD28 families has revealed additional pathways that provide costimulatory and inhibitory second signals to T cells. One of the newer pathways is represented by the programmed death 1 (PD-1; also known as CD279) receptor and its ligands, PD-L1 (B7-H1; CD274) and PD-L2 (B7-DC; CD273). PD-1 is a member of the CD28/CTLA4 family that is expressed on activated, but not resting T cells (Nishimura et al. (1996) Int. Immunol. 8:773). Ligation of PD-1 by its ligands mediates an inhibitory signal that results in reduced cytokine production, and reduced T cell survival (Nishimura et al. (1999) Immunity 11:141; Nishimura et al. (2001) Science 291:319; Chemnitz et al. (2004) J. Immunol. 173:945).
PD-L1 is a B7 family member that is expressed on many cell types, including APCs and activated T cells (Yamazaki et al. (2002) J. Immunol. 169:5538). PD-L1 binds to both PD-1 and B7-1. Both binding of T-cell-expressed B7-1 by PD-L1 and binding of T-cell-expressed PD-L1 by B7-1 result in T cell inhibition (Butte et al. (2007) Immunity 27:111). There is also evidence that, like other B7 family members, PD-L1 can also provide costimulatory signals to T cells (Subudhi et al. (2004) J. Clin. Invest. 113:694; Tamura et al. (2001) Blood 97:1809).
PD-L2 is a B7 family member expressed on various APCs, including dendritic cells, macrophages and bone-marrow derived mast cells (Zhong et al. (2007) Eur. J. Immunol. 37:2405). APC-expressed PD-L2 is able to both inhibit T cell activation through ligation of PD-1 and costimulate T cell activation, through a PD-1 independent mechanism (Shin et al. (2005) J. Exp. Med. 201:1531). In addition, ligation of dendritic cell-expressed PD-L2 results in enhanced dendritic cell cytokine expression and survival (Radhakrishnan et al. (2003) J. Immunol. 37:1827; Nguyen et al. (2002) J. Exp. Med. 196:1393). The structure and expression of PD-1, PD-L1, and PD-L2, as well as signaling characteristics and functions of these molecules in the context of regulating T cell activation and tolerance (e.g., therapeutic effects) are reviewed in greater detail in Kier et al. (2008) Ann. Rev. Immunol. 26:677, which is herein incorporated by reference in its entirety. Manipulation of this and other costimulatory pathways offers great potential to stimulate or suppress immune responses in humans and a need exists for compositions and methods useful for effecting such manipulations.